1. Field of the Invention
The present invention relates to a gas flow rate and temperature measuring element for measuring a flow rate of a fluid being measured, and relates to a gas flow rate and temperature measuring element used in a flow rate sensor for measuring an intake air flow rate of an internal combustion engine in an automobile, for example.
2. Description of the Related Art
Generally, in an automotive engine, etc., an air-fuel mixture including fuel and intake air is burnt in a combustion chamber in the engine body, and rotational output from the engine is extracted from the resulting combustion pressure, requiring that the intake air flow rate be detected in order to calculate the injection rate, etc., of the fuel with high precision.
Thermosensitive flow rate meters such as that described in Japanese Patent Laid-Open No. SHO 60-36916 (Gazette) or in “Intake Air Measurement Techniques for Automotive Engines” (The Transactions of the Institute of Electrical Engineers of Japan, pp 300-303, Vol. 118-E, No. 6, June, 1998), for example, are known as conventional techniques of this kind. These conventional thermosensitive flow rate meters are constructed such that a thermosensitive resistor for detecting a rate of air flow by means of heat transfer changes and an air temperature compensation resistor for compensating for air temperature at that time are each supported on tips of two electrically-conducting supports inserted through and supported by a holder. The holder of these conventional thermosensitive flow rate meters is mounted to a passage such that the thermosensitive resistor and the air temperature compensating resistor are disposed inside the passage to measure the flow rate of air flowing inside the passage.
In a conventional air flow rate measuring apparatus described in Japanese Patent Laid-Open No. HEI 8-219838 (Gazette), there is provided a thermosensitive resistor supported by a support pin disposed inside a subpassage disposed in a holder; and an air temperature compensating resistor supported by a support member which is separate from the holder. The holder and the support member of this conventional air flow rate measuring apparatus are mounted to a main passage such that the subpassage and the air temperature compensating resistor are disposed inside the main passage to measure the flow rate of air flowing inside the main passage.
However, in the conventional thermosensitive flow rate meters, because the thermosensitive resistor and the air temperature compensating resistor are supported by different electrically-conducting supports, one problem has been that the number of parts is increased, preventing reductions in parts costs and assembly costs. In addition, it is desirable for the thermosensitive resistor and the air temperature compensating resistor to be disposed in close proximity at a location at which the air flows stably, but because the thermosensitive resistor and the air temperature compensating resistor are supported by different electrically-conducting supports, another problem has been that there is a limit to how close the two can be disposed, making the air flow rate detecting precision poor.
Because the thermosensitive resistor and the air temperature compensating resistor are also supported by separate members in the first conventional air flow rate measuring apparatus, there are problems similar to those in the conventional thermosensitive flow rate meters.
A conventional gas flow rate and temperature measuring element capable of improving problems of this kind is described in Japanese Patent Laid-Open No. HEI 6-249693, for example. This conventional gas flow rate and temperature measuring element is constructed by adopting fine processing techniques such as etching techniques, thin-film film-formation techniques, etc., to form the thermosensitive resistor and the air temperature compensating resistor in close proximity on a single substrate made of silicon.
FIG. 9 is a top view showing a gas flow rate and temperature measuring element used in a conventional flow rate sensor, and FIG. 10 is a cross section taken along line X—X in FIG. 9 viewed from the direction of the arrows.
In the figures, an electrically-insulating support film 3 made of silicon nitride is formed on a front surface (a first surface) of a flat substrate 2 made of a single-crystal silicon, and a flow rate detector portion 4 and a gas temperature detector portion 5 are formed on the electrically-insulating support film 3 so as to be lined up in a direction perpendicular to a direction of flow A of a fluid being measured. Here, the gas temperature detector portion 5 is disposed near a first end portion of the flat substrate 2.
First to fourth flow rate detection electrode terminals 6a to 6d and first and second gas temperature detection electrode terminals 6e and 6f are formed on the electrically-insulating support film 3 near a second end portion of the flat substrate 2, first to fourth flow rate detection wiring 7a to 7d is formed on the electrically-insulating support film 3 so as to connect the first to fourth flow rate detection electrode terminals 6a to 6d and the flow rate detector portion 4, and first and second gas temperature detection wiring 7e and 7f is formed on the electrically-insulating support film 3 so as to connect the first and second gas temperature detection electrode terminals 6e and 6f and the gas temperature detector portion 5.
Furthermore, an electrically-insulating protective film 8 made of silicon nitride is formed by coating on the electrically-insulating support film 3 so as to cover the flow rate detector portion 4, the gas temperature detector portion 5, the first to fourth flow rate detection wiring 7a to 7d, and the first and second gas temperature detection wiring 7e and 7f. 
First and second cavities 9 and 10 having a trapezoidal cross-sectional shape are formed under the flow rate detector portion 4 and the gas temperature detector portion 5, respectively, by partially removing the flat substrate 2 from a rear surface (a second surface) of the flat substrate 2 by alkali etching. Thus, the flow rate detector portion 4 and the gas temperature detector portion 5 have a diaphragm construction, reducing the heat capacity of the flow rate detector portion 4 and the gas temperature detector portion 5, enabling them to respond sensitively to changes in the flow rate and the temperature of the gas.
Here, the flow rate detector portion 4, the gas temperature detector portion 5, the first to fourth flow rate detection wiring 7a to 7d, and the first and second gas temperature detection wiring 7e and 7f are formed by using photoengraving techniques and etching techniques to pattern a platinum film constituting a thermosensitive resistor film formed on the electrically-insulating support film 3. The flow rate detector portion 4 and the gas temperature detector portion 5 are formed into pectinate (comb-like) patterns, the first to fourth flow rate detection wiring 7a to 7d being formed so as to connect each of the first to fourth flow rate detection electrode terminals 6a to 6d and end portions of the pectinate patterns of the flow rate detector portion 4 generally linearly, and the first and second gas temperature detection wiring 7e and 7f each being formed so as to pass by a side portion of the flow rate detector portion 4 and connect the first and second gas temperature detection electrode terminals 6e and 6f and end portions of the pectinate pattern of the gas temperature detector portion 5. This first and second gas temperature detection wiring 7e and 7f is provided with: first wiring portions 11 wired parallel to and in close proximity to the flow rate detector portion 4; and second wiring portions 12 extending from these first wiring portions 11 to the end portions of the pectinate pattern of the gas temperature detector portion 5.
A conventional gas flow rate and temperature measuring element 1 constructed in this manner is disposed such that the electrode terminals 6a to 6f are electrically connected to a control circuit (not shown) by means of bonding wires (not shown) and a direction of alignment between the flow rate detector portion 4 and the gas temperature detector portion 5 is perpendicular to the direction of flow A of air constituting the fluid being measured. The temperature of air flowing over the electrically-insulating protective film 8 is detected by means of the gas temperature detector portion 5.
A heating current is allowed to flow through the flow rate detector portion 4, heating the flow rate detector portion 4. The heat generated in the flow rate detector portion 4 is transferred to the air flowing over the flow rate detector portion 4, reducing the temperature of the flow rate detector portion 4. If the flow rate of the air is high, the quantity of heat transferred to the air from the flow rate detector portion 4 increases, increasing the reduction in the temperature of the flow rate detector portion 4. On the other hand, if the flow rate of the air is low, the quantity of heat transferred to the air from the flow rate detector portion 4 is reduced, making the reduction in the temperature of the flow rate detector portion 4 small.
The heating current flowing through the flow rate detector portion 4 is controlled by the control circuit such that the average temperature of the flow rate detector portion 4 is higher than the temperature of air detected by the gas temperature detector portion 5 by a predetermined amount. Thus, the heating current flowing through the flow rate detector portion 4 is a function of the flow rate of the air, the flow rate of the air being detected by extracting this heating current as an air mass flow rate signal. Consequently, in this kind of flow rate sensor, it is extremely important that the temperature of the air flowing over the flow rate detector portion 4 is measured accurately for the flow rate of the air to be detected accurately.
In the conventional gas flow rate and temperature measuring element 1 constructed in this manner, because the flow rate detector portion 4 and the gas temperature detector portion 5 are disposed on the flat substrate 2, the flow rate detector portion 4 and the gas temperature detector portion 5 are supported by a single member, enabling reductions in the number of parts, thereby enabling reductions in parts costs and assembly costs. Because the flow rate detector portion 4 and the gas temperature detector portion 5 can be disposed in close proximity to each other, reductions in size are enabled and the gas temperature detector portion 5 can detect the temperature of the air flowing over the flow rate detector portion 4. Thus, the average temperature of the flow rate detector portion 4 can be controlled so as to be higher by a predetermined amount than the temperature of the air flowing over the flow rate detector portion 4, improving the precision in detecting the flow rate of the air.
However, because portions of the first and second gas temperature detection wiring 7e and 7f connecting the first and second gas temperature detection electrode terminals 6e and 6f and the end portions of the pectinate pattern of the gas temperature detector portion 5, in other words, the first wiring portions 11, are disposed parallel to and in close proximity to the flow rate detector portion 4, heat generated in the flow rate detector portion 4 is conducted through the electrically-insulating support film 3 and the electrically-insulating protective film 8 to the first wiring portions 11, and in addition, heat generated in the flow rate detector portion 4 is transferred through the air to the first wiring portion 11 disposed downstream from the flow rate detector portion 4. Because the heat conveyed to the first wiring portions 11 is conducted through the second wiring portions 12 to the gas temperature detector portion 5, one problem has been that the gas temperature detector portion 5 can no longer detect the temperature of the air accurately, making the precision in detecting the flow rate of the air poor.
Particularly when the gas flow rate and temperature measuring element 1 are reduced in size, the distance between the first wiring portions 11 and the flow rate detector portion 4 cannot be adequately ensured, making the above-mentioned deterioration of the precision in detecting the flow rate of the air pronounced.